The present invention relates to a process and a device for measuring through a holographic technique the deviation of light rays in transparent media traversed by a laser beam, and notably of inhomogeneous objects whose refraction coefficient may show strong variation gradients.
The process and the device according to the invention notably apply to the study of the variations of the refraction coefficient of inhomogeneous substances (gaseous, liquid or solid), in order to know some of their properties and notably their composition, their concentration or their temperature. An interesting particular application of the present invention is, for example, the study of turbulent mixtures in the combustion chambers of engines. The study of the properties of transparent objects is generally carried out with optical means by using the property of any transparent object to more or less deviate the light rays traversing it according to variations of its volume and of its refraction coefficient.
There are several well-known deflectometry methods for carrying out these studies. Techniques such as holographic deflectometry, holographic moire, single-beam deviation measurings or speckle pattern photography can be used. The holographic moireand the speckle pattern photography techniques are similar insofar as they allow recording of the deviation of the light for a later analysis. Their resolution in time is therefore high. With the moiretechnique, only one component, of the deviation of the deviated beam can be measured at a time, and the experimenter must first set the sensitivity and the space resolution. The moiretechnique by holography does not have these drawbacks, but the data reduction, that is to say the extracting of the data of interest, is relatively complicated. The technique of speckle pattern photography allows measurement of simultaneously measure the two components of the deviated beam, but a high sensitivity can only be obtained to the detriment of the space resolution, and the values of these two parameters must be set before beginning the measuring. This particular technique is generally utilized for studying objects whose variation gradient of the refraction coefficient is relatively low.
It is also well-known that the moireor the speckle pattern photography techniques are not applicable when the variations in the refraction coefficient of the examined objects are large enough to cause a crossing of the light rays before their deviation has been measured. The single-beam deviation technique also avoids the drawbacks of the moireor of the speckle pattern photography techniques. No light ray crossing occurs when it is used, but it is not suitable for the study of transient phenomena.
Applications of these methods known for the study of the variations in the refraction coefficient of transparent objects or media are described in the following publications:
J. Stricker et al, "Holographic MoireDeflectometry" in Appl. Phys. Lett. Vol. 44 No. 8, 723-725; PA0 U. Koepf, "Applications of Speckling for measuring the deflection of laser light by phase objects" in Opt. Comm. Vol. 5, No. 5, 347-350.